It is well known that most liquid propellants provide higher lsp than do solid propellants, and thus are more energy efficient than solid propellants. Liquid propellant rocket engines can be throttled to control the thrust, and in some applications can be stopped and restarted. However for most space booster applications in the payload class of 500 to 6000 to 20,000 pounds to Low Earth Orbit (LEO), solid rocket motor space boosters can be as reliable and less costly than liquid propellant boosters.
The inherent simplicity of solid rocket motor boosters reduces the processing times and headcount. Liquid propellant boosters inherently require more complex processing procedures resulting in longer processing times and a larger workforce at the launch site. Thus the processing component of cost, for solids, is substantially less.
The U.S. Military became the driving force behind the development of solid propellant missiles. The military requirements were, however, substantially different. They emphasized high performance, quick reaction, simplicity, and safety in operations. There were stringent weight, length, and volume constraints, especially for submarine based missiles.
Likewise, because of the always increasing military requirements for additional performance within weight and dimension constraints, the missile designers were forced to lighter-weight structures, propellants with higher lsp, reducing inerts to increase the mass fraction of the booster stages, and trimming design and manufacturing margins to the minimum. These measures required additional tests during development and production, more inspections, and certainly a proliferation of documentation verifying the work. All at added expense.
On the other hand, these requirements also spawned substantial advances and developments in materials, ever increasing strength to weight ratios. Metal cases were replaced by glass to be replaced by Kevlar in turn to be replaced by graphite-epoxy filament wound cases of ever increasing strength as the technology evolved.
Graphite nozzle throats became carbon-carbon which improved reliability although performance also increased due to reduced throat erosion.
It was recognized that if performance driven requirements such as booster weight and volume constraints could be relaxed, substantial improvements in reliability and lower manufacturing cost could be realized.
It was decided that users' requirements could be met with substantial margin--for spacecraft weight and volume growth during development--by combinations of existing or about to become existing solid rocket motors, and, as an example, using robust aluminum structure for interstages and equipment sections rather than composites.
The approach proposed is to minimize redundant handling. There are three major activities. The first is solid rocket motor receiving at the launch facility; installation of destruct ordnance, and vertical stacking at the launch pad. The payload (spacecraft) which is previously checked out in a standard facility is installed on the equipment section at that facility. Any payload specific consumables (for example hydrazine for spacecraft propulsion) is loaded at this facility. The fairing (shroud) is installed. The complete assembly is transported to the launch pad, and stacked on the booster. This stacking process is limited to electrical connections, and mechanical mate. Environmental conditioning is provided to the spacecraft if required. The objective is to keep as much activity as possible in existing buildings in a shirt sleeve, floor level environment. This approach reduces cost by avoiding expensive gantry features such as a clean room. On-the-launch pad checks before the launch countdown are then limited to verifying connectivity.